Aromatic carboxylic acids are useful chemical compounds and are raw materials for a wide variety of manufactured articles. For example, terephthalic acid is manufactured on a world-wide basis in amounts exceeding 10 billion pounds per year. A single manufacturing plant can produce 100,000 to more than 750,000 metric tons of terephthalic acid per year. Terephthalic acid is used, for example, to prepare polyethylene terephthalate, a raw material for manufacturing polyester fibers for textile applications and polyester film for packaging and container applications. Terephthalic acid can be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock compound, such as para-xylene, in a liquid-phase reaction using air or other source of dioxygen (molecular oxygen) as the oxidant and catalyzed by one or more heavy metal compounds and one or more promoter compounds.
Methods for oxidizing para-xylene and other aromatic compounds using such liquid-phase oxidations are well known in the art. For example, Saffer in U.S. Pat. No. 2,833,816 discloses a method for oxidizing aromatic feedstock compounds to their corresponding aromatic carboxylic acids. Central to these processes for preparing aromatic carboxylic acids is employing an oxidation catalyst comprising a heavy metal component and a source of bromine in a liquid-phase reaction mixture including a low molecular weight monocarboxylic acid, such as acetic acid, as part of the reaction solvent. A certain amount of water is also present in the oxidation reaction solvent and water is also formed as a result of the oxidation reaction. Although various means can be used to control the temperature of the highly exothermic oxidation reaction, it is generally most convenient to remove heat by allowing the solvent to vaporize, i.e. boil, during the oxidation reaction. Gaseous effluent from the oxidation reaction generally comprises steam, monocarboxylic acid, an ester thereof, carbon dioxide, carbon monoxide and bromine which, depending on the aromatic feedstock compound used, is mainly in the form of one or more alkyl bromide compounds, such as methyl bromide. Methyl bromide is toxic and, if discharged into the atmosphere, is believed to contribute to depletion of atmospheric ozone. It is therefore important to avoid discharge of methyl bromide into the atmosphere. Additionally, when compressed air is used as the source of dioxygen, the gaseous effluent contains nitrogen gas and unreacted dioxygen.
The vaporized solvent, which is typically a mixture of water and low-molecular weight carboxylic acid, has heretofore been condensed in one or more overhead condenser apparatus and the condensate returned to the reaction mixture. However, since water is also present, at least part of the condensate is usually directed to a separation apparatus, typically a distillation column, to separate the water from the low molecular weight aliphatic acid solvent so that the water concentration in the reactor is maintained at a constant level. Constituents of the gaseous effluent that are not condensed are typically passed through a vapor phase oxidation unit to burn these volatile organic byproducts and form an environmentally acceptable effluent.
The high pressure offgas contains a considerable amount of energy. Although prior art processes have, to an extent, utilized some of the energy contained in the offgas by running the offgas through, for example, an expander or turbine, prior art processes did not fully utilize the energy available in this high pressure offgas. In prior processes, heat removal from the reaction mixture was accomplished by condensing a portion of the reaction overhead vapor to produce moderate pressure steam. The moderate pressure steam, in part, was used to recover energy by a steam turbine, and a part was used to separate water from acetic acid by distillation.
In WO 96/39595, International Application Number: PCT/GB96/01261, a process is proposed for energy recovery from effluent gas derived from production of terephthalic acid by subjecting the effluent gas to catalytic combustion to convert methyl bromide vapor to bromine and/or gaseous hydrogen bromide and passing the resulting gas stream through an energy conversion device such as a gas turbine under controlled conditions of pressure and temperature such that condensation of hydrogen bromide and/or bromine is prevented in the energy conversion device. Hydrogen bromide and bromine are potential corrosion-producing agents especially in a mixture with condensate. The application states that where the presence of any condensation of hydrogen bromide and/ or bromine is prevented in equipment downstream of the catalytic oxidation zone, such may be fabricated from relatively inexpensive materials. Following passage through the energy recovery system according to the application, however, the gas is contacted with a liquid in a scrubbing unit to reduce the hydrogen bromide and bromine content of the gas vented to the environment.
In U.S. Pat. No.5,113,015 and 5,235,102 to Palmer et al. processes are provided for recovering acetic acid from methyl acetate wherein the methyl acetate is hydrolyzed catalytically to methanol and acetic acid in the same tower or column that is used to separate the hydrolysis products. Advantageously, in the catalytic distillation a catalyst-packing material comprising a rigid, cellular monolith or a rigid, cellular monolith coated with a catalytically-active material is employed. Preferred rigid, cellular monoliths are ceramic honey-comb monoliths.
Significantly, U.S. Pat. Nos. 5,113,015 and 5,235,102, and WO 96/39595 make no reference to any possibility for recovery of hydrogen bromide from an alkyl bromide compound by hydrolyzing the alkyl bromide compound to hydrogen bromide and corresponding alcohol.
In the past various molecular sieve compositions natural and synthetic have, however, been found to be useful for a number of hydrocarbon conversion reactions. Among these are alkylation, aromatization, dehydrogenation and isomerization. Among the sieves which have been used are Type A, X, Y and those of the MFI crystal structure, as shown in "Atlas of Zeolite Structure Types," Second Revised Edition 1987, published on behalf of the Structure Commission of the International Zeolite Associates and incorporated by reference herein. Representative of the last group are ZSM-5 and AMS borosilicate molecular sieves.
Prior art developments have resulted in the formation of many synthetic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (Milton, in U.S. Pat. No. 2,882,243), Zeolite X (Milton, in U.S. Pat. No. 2,882,244), Zeolite Y (Breck, in U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (Argauer, et al., in U.S. Pat. No. 3,702,886), Zeolite ZSM-II (Chu, in U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (Rosinski, et al., in U.S. Pat. No. 3,832,449), and others.
The art, therefore, needs an improved method for handling alkyl bromide compounds that are produced as co-products during exothermic, liquid-phase oxidation of aromatic feedstock compounds to aromatic carboxylic acids.
It is desirable that an improved process shall recover from the alkyl bromide compounds the bromine as hydrogen bromide which is a form of bromine useful in the liquid-phase oxidation reaction mixture.
Furthermore, because hydrogen bromide and bromine are potential corrosion-producing agents especially in a mixture with condensate, an improved process whereby the recovered hydrogen bromide is recycled directly to the liquid-phase oxidation is particularly desirable.
Advantageously, such improvements shall be combined in a process for preparing aromatic carboxylic acids by the exothermic, liquid-phase oxidation of an aromatic feedstock compound wherein the energy produced by the exothermic oxidation is efficiently recovered, and uses of water produced during the preparation of aromatic carboxylic acids are efficiently integrated into the process.
The present invention provides such an improved process.